1. Field of the Invention
The present invention relates to a method for manufacturing a of semiconductor device having an SiN layer as an outermost protective surface layer, an interlayer insulator or the like.
2. Description of the Related Art
Because of its non-porosity and excellent moisture resistance, an SiN layer is used as a passivation layer of semiconductor device, i.e., as an outermost protective surface layer, an interlayer insulator or the like. In most cases SiN layers are formed by plasma CVD methods using as gaseous raw material either SiH.sub.4 +NH.sub.3 or SiH.sub.4 +N.sub.2, in order not to damage Al wirings or other structures already formed on the substrate.
FIG. 1 shows a prior art semiconductor device having an SiN layer as a passivation layer. As illustrated in FIG. 1A, an interlayer insulator 12 such as an SiO.sub.2 layer or the like is formed on a semiconductor substrate 11 such as an Si substrate or the like and Al wirings 13 are patterned on the interlayer insulator 12. Subsequently, by the above-mentioned plasma CVD method, there is formed an SiN layer 14 which thus covers the Al wirings 13 as illustrated in FIG. 1B.
With an increase in the integration level of semiconductor devices, however, patterns of the Al wirings 13 have undergone further miniaturization. This gives rise to, as illustrated in FIG. 1B, a degradation in the step coverage of the SiN layer 14 and, in consequently, voids may form in the SiN layer 14 between the Al wirings 13. Accordingly, a reliable semiconductor device is not provided by the method under these circumstances.
An approach has been proposed to improve the step coverage of the SiN layer using sputter effect by an increased ion impact of low energy during the deposition of the SiN layer. In accordance with this method, a low frequency voltage is applied, to the electrode on which a semiconductor substrate is placed, and a high frequency voltage is applied to the other electrode in a parallel plate plasma CVD apparatus. By this approach, an improvement in the step coverage has been achieved but not to such an extent as to form a conformal SiN layer.
For this reason, another approach using plasma CVD methods has been proposed which includes as gaseous raw materials an organic Si compound containing nitrogen, instead of an SiH.sub.4 type gaseous raw material. An SiN layer excellent in the step coverage can be formed using an organic Si compound containing nitrogen, by virtue of an intermediate product formed, during the formation of the above SiN layer, which polymerizes and exhibits fluidity.
Although SiN layer formed by plasma CVD methods using an organic Si compound containing nitrogen as a raw material is excellent in terms of step coverage, its dielectric constant is high. A concrete example will be explained which reference to FIG. 2. An interlayer insulator 12 such as an SiO.sub.2 layer or the like is formed on a semiconductor substrate 11 such as an Si substrate or the like. Next, first Al wirings 13,13' are formed by patterning on the interlayer insulator 12. Then, on the Al wirings 13,13', there is formed by plasma CVD methods an SiN layer 14 which thus covers the Al wirings 13,13'. Thereupon, second Al wiring 15 is formed by patterning on the SiN layer 14. Consequently, as illustrated in FIG. 2, a coupling capacitance C.sub.2 is formed between the first wirings 13 and 13', and an interlayer capacitance C.sub.1 is formed between the first wirings 13,13' and the second wiring 15.
If an SiN layer is used as an interlayer insulator, both the above coupling capacitance C.sub.2 and the interlayer capacitance C.sub.1 are large in their values due to the high dielectric constant of the layer. Therefore, delay in operation gets larger due to the large parasitic capacitances, i.e., it is difficult in this way to produce a semiconductor device which is fast in operation. Moreover, due to the larger parasitic capacitances, a semiconductor device having an SiN layer as an interlayer insulator consumes during its operation a larger amount of electric power. If such semiconductor devices are operated by cells or the like, operation time of the cells becomes shorter which is problematic.